The present invention relates to an amplifier for amplifying radio frequency (RF) signals. The invention has particular but not exclusive relevance to delta-sigma amplifiers for wireless communications systems and devices thereof operating according to the Long Term Evolution (LTE) technologies defined by the 3rd Generation Partnership Project (3GPP).
Modern communications standards such as 3G and LTE employ modulation schemes which require linear RF amplifiers. The term ‘linear’ refers to the ability of an amplifier to produce output signals that are accurate copies of the input signal (generally) at increased power levels. Linear amplification is essential for non-constant envelope signals such as OFDM to prevent the creation of unwanted in-band interference signals. Linear amplification must be achieved for peak signal power, not just for average signal power. Peak signal power can be up to 10 dB higher than average signal power for OFDM signals.
Linearity is usually achieved by backing-off the amplifier's power level to below its maximum (and most efficient) region to a region exhibiting a more linear amplification for both the average and the expected maximum signal level. However, this effectively results in a reduction of the amplifier's overall power efficiency compared to the case when the amplifier operates near its peak power level most of the time. Consequently, linear RF amplifiers typically have a power efficiency of less than 10%.
Various techniques have been employed to achieve more power-efficient linear RF amplifiers, which techniques include, for example, pre-distortion, envelope elimination and restoration, cartesian feedback, and/or the like.
A relatively new approach for efficient linear RF amplification is the use of a so-called S-class amplifier, which uses delta-sigma modulation to generate directly an amplified RF signal. Since S-class amplifiers use field effect transistors (e.g. metal-oxide-semiconductor, ‘MOS’ transistors) (or other transistors) for generating a modulated signal, and since transistors are either turned on or off, efficiency of S-class amplifiers can theoretically approach 100%.
However, a key problem with S-class amplifiers is how to generate a delta-sigma modulation signal that is fast enough to generate an amplified RF signal. This problem arises because the delta-sigma modulation signal frequency (also referred to as the delta sigma bitstream rate) must be at least twice (typically four times) the carrier frequency of the signal to be amplified (due to the Nyquist sampling theorem). It is difficult to compute the delta-sigma modulation signal at high speed because delta-sigma modulation has a single-cycle feedback loop, which consists of several steps including: summation of the input signal with a loop-filtered error signal; quantisation (usually as truncation of a fixed point binary number); and error feedback into a loop filter. The logic in the delta-sigma modulator must be computed at the delta sigma bitstream rate. Since cellular signals may be transmitted at around 1 GHz (typically in the range of 800-900 MHz for applications such as LTE) or even above 1 GHz, the delta-sigma modulation signal must be generated at several GHz. This effectively means that the delta-sigma modulator logic must operate at an impractically high rate, which would inhibit feasible and/or economical implementation of delta-sigma amplification for radio frequency signals used in LTE systems and/or the like.